Platinum distribution in soil profiles of the Tulameen ultramafic complex, southern British Columbia

Abstract

Exploration for chromitite-associated Pt deposits is hampered by a poor understanding of the distribution and behaviour of Pt in the surficial environment. This study investigates Pt content, residence sites and PGE mineralogy of soils developed on till and colluvium associated with the Tulameen ultramafic complex in southern British Columbia. Seventy-six soil profiles, as well as sediments, bogs and waters were sampled above the dunite core of the Tulameen complex, within which Pt occurrences consist of massive-to-discontinuous segregations of platiniferous chromitite. Platinum content of the −70 mesh fraction of soils and sediments was determined by lead fire assay-inductively coupled plasma emission spectrometry. Samples from fourteen selected profiles were then examined in detail to determine Pt mineralogy and its distribution between different size, density and magnetic fractions. Platinum concentrations in the −70 mesh fraction of C horizon soils range from 2 to 885 ppb and are closely related to soil dunite content, as estimated from MgO content and verified by XRD mineralogy. Dunite colluvium (mean: 24.2% MgO), locally-derived dunitic till (mean: 16.5% MgO) and exotic non-dunitic till (mean: 5.7% MgO) have median Pt concentrations of 88 ppb, 36 ppb and 8 ppb, respectively. This trend is evident in all size and density fractions. Platinum content of heavy mineral (SG > 3.3) fractions is 10–20× greater than in light mineral fractions. Platinum is most abundant in the heavy magnetic fraction from non-dunitic tills and dunitic tills remote from known mineralization, but the proportion of Pt in the heavy non-magnetic fraction increases with increasing proximity to mineralization. Scanning electron microscopy and microprobe studies of heavy fractions from C-horizons identified Pt-Fe-Cu alloys as free grains, and as inclusions in Mg-silicates and chromites. Chromite occurs as Mg-Cr-rich anhedral fragments and as Fe-rich euhedral to subhedral crystals. The latter are relatively more abundant in the magnetic fraction and are interpreted as Pt-poor grains disseminated throughout the dunite. The Mg-Cr-rich chromite fragments are relatively more important in the non-magnetic fraction and are interpreted as remnants of Pt-bearing massive chromitite segregations. The abundance of chromite fragments in soils near chromitite segregations accounts for the high Pt content of the non-magnetic heavy fractions of these soils. The −270 mesh fraction or the magnetic heavy fraction of C-horizon soils would be the most suitable sample media for reconnaissance geochemical sampling. However, the greater contrast in Pt concentrations, more limited dispersal and Mg-Cr-rich chromite association of the non-magnetic heavy fraction make it a more suitable medium for detailed geochemical sampling.